This invention concerns a method of ensuring quality of information transfer of an information packet, when the information packet is protected by coding and interleaved into at least two transmission periods, in radio bursts in a system implemented with TDMA technology or in power regulation periods in a system implemented with CDMA technology, for example.
In the transfer of speech and data in a digital telecommunications system, such transfer errors occur in the transfer link which reduce the quality of the transferred signal. Through error correction of the digital signal to be transmitted, e.g. through channel coding and/or retransmission, and through interleaving of bits the quality of the transmission and the transfer error tolerance are improved. In channel coding, such redundant data is added to the data to be transmitted, with the aid of which the original data can be detected faultlessly in the receiver, even if the signal would corrupt during the transfer. Channel coding may be carried out e.g. as convolution coding or as repetition coding. Retransmission is used for correcting transfer errors either independently or e.g. in addition to channel coding, whereby any errors in a channel-coded transmission are corrected by a retransmission of the distorted frames. In interleaving of bits to be transmitted, the bits of several code words are mixed together, whereby adjacent bits of the signal will spread out into several radio bursts. FIG. 2 illustrates interleaving of speech bursts S1, S2 and S3 into radio bursts R1-R6. In the example shown in the figure, each speech burst S1-S3 is interleaved into four successive radio bursts. Owing to the interleaving it may still be possible to detect the signal, even if a whole radio burst were to be lost in the information transfer.
FIG. 1 in the appended drawing is a simplified block diagram of the GSM system (Global System for Mobile communications). A mobile Station (MS) is connected over a radio path with some base transceiver station (BTS), in the case shown in FIG. 1 with base transceiver station BTS1. The base station sub-system (BSS) includes a base station controller (BSC) and subordinated base transceiver stations BTS. Subordinated to a mobile services switching centre (MSC) are usually several base station controllers BSC. The mobile services switching centre is connected to other mobile services switching centres, and through the gateway mobile services switching centre (GMSC) the GSM network is connected with other networks, such as public switched telephone network PSTN, another public land mobile network PLMN or ISDN network. The whole system is monitored by an Operation and Maintenance Centre OMC.
In a mobile communications system, transmission power control is performed in mobile station MS and/or in base transceiver station BTS in order to reduce the network""s noise level and compensate for fading on the radio path. Power control generally aims at preserving the received signal constantly almost at the same power level, which is as low as possible, however, so that the quality of the received signal is preserved. When the signal and/or power level in a radio communication between the mobile communications network and the mobile station drops below the desirable level, control of the transmission power may be preformed at base transceiver station BTS and/or in mobile station MS in order to improve the radio communication. The transmission power of mobile station MS is usually controlled from a fixed network with the aid of a special power control algorithm. Mobile station MS measures the reception lever (field strength) and quality of the downlink signal reveived from ase transceiver station BTS1 of the serving cell, whereas base treansceiver station BTS1 of the serving cell measures the reception level (field strength) and quality of the uplink signal received from mobile station MS. Based on these measurement results and on established power control parameters, the power control algorithm determines a suitable transmission power level, which is then made known to mobile station MS in a power control command. Power control is performed continuously during the call, e.g. in a GSM system of the TDMA type typically twice a second and in a UMTS-WCDMA system of the CDMA type (Universal Mobile Telecommunication System) 1600 times a second.
In radio systems implemented with Time Division Multiple Access (TDMA) technology, the signal is transferred over the radio path in radio bursts, some of which are reserved for trafficking use, e.g. for transmitting speech bursts or user data. At times such a traffic channel may be stolen for other than trafficking use, e.g. for signaling, when an extra need for signaling occurs unexpectedly (in-burst signalling). Hereby the information which is intended for transmission in the stolen radio burst will not be transmitted, but owing to coding and interleaving of the information it may be possible from the received radio bursts to interpret the information to be transferred. However, the likelihood that the information is lost completely, e.g. that a whole speech burst is lost, will grow considerably as the information in the end of the information packet is subjected to the effect of transfer errors on the transfer path.
In code division multiple access (CDMA) radio systems, the function is based on spread spectrum communication. The data signal to be transmitted is multiplied by a special hash code, whereby the transmission will spread onto a wide-band radio channel. Hereby several users may use the same wide-band radio channel at the same time for transmitting CDMA signals processed by different hash codes. In CDMA systems, the special hash code of each subscriber will hereby produce a traffic channel in the system, in the same sense as a time slot in TDMA systems. When required, one or more power control periods of a traffic channel may be stolen for signalling use, whereby the information which was to be transmitted in these power control periods will not be transmitted. The stealing of power control periods for other use causes an increased probability of loss of the information packet, like the stealing of radio bursts in a TDMA system.
Discontinuous Transmission (DTX) means a functionality where the transmission of a mobile station or a base transceiver station on the radio path can be cut off, when the signal to be transmitted does not contain any information significant to the recipient, e.g. for the duration of pauses in speech. The purpose of this is to reduce the transmitter""s consumption of current, which is very essential for a mobile station, and to lower the noise level of the network. Discontinuous transmission is generally known in connection with digital mobile communications systems. The speech activity of the signal to be transmitted is monitored in the mobile station and in the base transceiver station, and the transmission to the radio path is cut off when there is no speech information. When the speech begins again, the speech is coded and transmitted to the radio path in the proper time slot. E.g. in a radio system according to FRAMES FMA1 and implemented with WB-TDMA (Wide Band Time Division Multiple Access) technology, the WB-BETH protocol allows transposition of users, that is, the connection is cut off and the radio bursts are taken over for other use while the transmission is cut off, and the connection is again quickly set up, when the speech reoccurs. Hereby the user does not normally notice from any reduced speech quality that the channel becomes free. However, such situations become a problem, where the connection can not be re-established so quickly as required, e.g. when momentarily there is no free channel at the base transceiver station for transmission of radio bursts, or when signalling commands have collided on the radio path. Hereby the information of one or more radio bursts is lost, when radio bursts can not be transmitted. Even the non-transmission of one radio burst will result in a considerably increased probability of loss of partly lost speech bursts, because other transfer errors will result on the transfer path, besides non-transmission.
Thus, the problem in the above-mentioned information transfer situations is the loss of parts of speech bursts contained in the radio burst or in the power control period, and thus the considerably increased probability of loss of the speech bursts in question. In addition, the quality of speech may then suffer.
The purpose of this invention is to reduce the probability of loss of coded and interleaved information packets, especially when one or more transmission periods are not transmitted for some reason.
This objective is achieved by methods according to the invention, which are characterised by what is said in the independent claims 1 and 8. Special embodiments of the invention are presented in the dependent claims.
The invention is based on the idea that the transmitter increases the transmission power for at least one transmission period containing at least a part of an information packet, a part of which has not been transmitted. The increase of transmission power is performed when at least one transmission period is not transmitted. When information packets are coded and interleaved between several transmission periods, an allocated increase of the transmission power hereby reduces transfer errors in these transmission periods to be sent and makes it less likely that information packets are lost entirely or that the quality of the information transfer will suffer. Lost information is patched by increasing the transmission power. In another method of embodying the invention, the increase of transmission power is done for at least one transmission period when at least one transmission period is not transmitted and when the increase of transmission power allows rescuing of the information packet.
In this application, a transmission period means a coherent period during which power control can not be performed, that is, a period between two successive power controls, when power control is performed at the maximum frequency. The transmission period includes one radio burst in a TDMA system and one power control period in a CDMA system.
It is an advantage of the method according to the invention that the release of a channel, e.g. for the time of a speech pause, and the theft of a radio burst for other use cause less of a risk of reduced speech quality.
It is another advantage of the method according to the invention that it will not cause any extra traffic or signalling on the transfer path.
In addition, it is an advantage of the method according to the invention that it is possible with its aid quickly to compensate for any increased probability of a loss of an information packet.
The method also achieves the advantage that the need for retransmission is reduced, whereby the capacity of the cell will increase.
It is also an advantage of the method according to the invention that any increase of transmission power can be targeted at a real need, because the transmitter knows for sure that reception is not possible, since the transmission period was not sent in the first place.